1. Field of the Invention
The present invention relates to a temperature compensated crystal oscillator. Particularly, the present invention relates to a temperature compensation method for a crystal oscillator that is used as a reference frequency in a mobile communication and a satellite communication etc.
2. Description of the Related Art
In recent years, the utilization range of the land mobile communication as represented by portable telephones is expanding steadily. Along this development, the portable telephones are also spreading rapidly, and are involving severe technical development competitions. Crystal oscillators that are used in the portable telephones are also required to have higher performance at low cost in small sizes. The oscillation frequency of the crystal vibrator changes in a cubic curve versus a change in the environmental temperature, as shown in FIG. 23. In order to obtain high stability, an oscillation circuit is provided with a temperature compensation circuit to offset the temperature characteristics of the vibrator. The temperature compensation method is either a direct temperature compensation method or an indirect temperature compensation method. It is general that any one of these method changes the load capacitance of the oscillation circuit thereby to compensate for temperature. The oscillators used in a PLL circuit (phase-locked loop circuit) are required to have a function of changing the frequency by applying a voltage (Vcont) from the outside. An oscillator used in a PLL circuit for a satellite communication is required high stability, therefore, not only Vcont function but also a temperature compensated function (circuit) is provided in the oscillator. In other words, two or more variable functions are provided within the oscillation circuits to change the load capacitance and change the frequency. These functions necessarily interfere with the variable ranges. For example, assume that 100 ppm of the frequency control range is necessary for the temperature compensation circuit. In this case, when 20 ppm of the frequency control range is externally changed by Vcont, a ratio of the load capacitance of the temperature compensation circuit to a total load capacitance changes. Therefore, a frequency change due to the change of the load capacitance changes, which results in 99 ppm of the frequency control range by a temperature compensation circuit, and this aggravates the temperature characteristics by 1 ppm.
FIG. 24 is a block diagram of a conventional temperature compensation type crystal oscillator having an external variable capacitance function (Vcont). A temperature compensation voltage generator 116 generates a compensation functional voltage, and applies the voltage to a variable capacitance diode 114. A load capacitance of the oscillation circuit changes based on a change in the capacitance of the diode. Accordingly, the temperature characteristics of the oscillation frequency of a crystal oscillator 111 are controlled to be flat. Consequently, the frequency temperature characteristics of the oscillator become excellent. In this case, a voltage is input from an external variable controller (Vcont) 117, and is applied to a variable capacitance diode 115 thereby to change the frequency. This not only changes the oscillation frequency but also affects the temperature compensation quantity.
The temperature compensation quantity and the external variable quantity have a strong relationship with a capacitance ratio of the oscillator (γ=Co/C1) as follows.
1. Basic Theory
An equation (1) expresses an offset frequency deviation from a series resonance frequency when the crystal oscillator oscillates.                               …          ⁢                                          ⁢                      D            L                          =                              1            2                    ×                                    C              1                                      (                                                C                  0                                +                                  C                  L                                            )                                                          (        1        )                            DL: Oscillation frequency deviation        C1: Series capacitance of a vibrator        C0: Parallel capacitance (Inter-electrode capacitance) of a vibrator        Cc: Capacitance of a circuit        
FIG. 25 illustrates an oscillation frequency equivalent block diagram expressed by the equation (1).
FIG. 26 illustrates isolation of the CL into three series quantities of Cx, Cy and Cc as given by an expression (2).                               …          ⁢                                          ⁢                      1                          C              L                                      =                              1                          C              x                                +                      1                          C              y                                +                      1                          C              c                                                          (        2        )            
For example, Cx represents a temperature compensation quantity, Cy represents a frequency adjustment or an external variable capacitance, and Cc represents an oscillation circuit capacitance. When Co=0 (Open), the following equation (3) can be obtained.                               D          L                =                                            C              1                                      2              ⁢                              C                L                                              =                                                                      C                  1                                2                            ⁢                              (                                                      1                                          C                      x                                                        +                                      1                                          C                      y                                                        +                                      1                                          C                      c                                                                      )                                      =                                                            C                  1                                                  2                  ⁢                                      C                    x                                                              +                                                C                  1                                                  2                  ⁢                                      C                    y                                                              +                                                C                  1                                                  2                  ⁢                                      C                    c                                                                                                          (        3        )            
As shift quantities of the frequency from the series resonance frequency are added to the capacitances of Cx, Cy, and Cc respectively, the frequency deviation is not interfered with suppression or the like due to the respective capacitances change.
However, as a piezoelectric element such as the crystal vibrator requires an electrode to promote oscillation, the inter-electrode capacitance Co cannot be omitted.
The following equation (4) is obtained from the equations (1) and (2).                                                                                           D                  L                                =                                                                            C                      1                                                              2                      ⁢                                              (                                                                              C                            0                                                    +                                                      C                            L                                                                          )                                                                              =                                                            C                      1                                                              2                      ⁢                                                                        C                          0                                                ⁡                                                  (                                                      1                            +                                                                                          C                                L                                                                                            C                                0                                                                                                              )                                                                                                                                                                                            =                                                      1                                          2                      ⁢                                              γ                        (                                                  1                          +                                                      1                                                                                          C                                0                                                                                            C                                L                                                                                                                                    )                                                                              =                                      1                                          2                      ⁢                                                                                          ⁢                                              γ                        (                                                  1                          +                                                      1                                                                                          1                                x                                                            +                                                              1                                y                                                            +                                                              1                                c                                                                                                                                    )                                                                                                                                                                    =                                                      1                                          2                      ⁢                                                                                          ⁢                      γ                                                        ×                                                            x                      +                      y                      +                                                                        x                          ⁢                                                                                                          ⁢                          y                                                c                                                                                    x                      +                      y                      +                                              x                        ⁢                                                                                                  ⁢                        y                                            +                                                                        x                          ⁢                                                                                                          ⁢                          y                                                c                                                                                                                                ⁢                                  ⁢                                            D              L                        =                                          1                                  2                  ⁢                                                                          ⁢                  γ                                            ⁢                              F                ⁡                                  (                                      x                    ,                    y                    ,                    c                                    )                                                              ,                                          ⁢                                    …              ⁢                                                          ⁢              D              ⁢                                                          ⁢                              S                x                                      =                                          1                                  2                  ⁢                  γ                                            ⁢                                                ⅆ                  F                                                  ⅆ                  x                                                                                        (        4        )                                                                                                      F                  ⁡                                      (                                          x                      ,                      y                      ,                      c                                        )                                                  =                                                      x                    +                    y                    +                                                                  x                        ⁢                                                                                                  ⁢                        y                                            c                                                                            x                    +                    y                    +                                          x                      ⁢                                                                                          ⁢                      y                                        +                                                                  x                        ⁢                                                                                                  ⁢                        y                                            c                                                                                  ,                                                          ⁢                                                …                  ⁢                                                                          ⁢                                      S                    x                                                  =                                                      ⅆ                    F                                                        ⅆ                    x                                                                                                                          =                                                                    -                                          c                      2                                                        ⁢                                      y                    2                                                                                        {                                                                  x                        ⁢                                                                                                  ⁢                        y                                            +                                              c                        ⁡                                                  (                                                      x                            +                            y                            +                                                          x                              ⁢                                                                                                                          ⁢                              y                                                                                )                                                                                      }                                    2                                                                                        (        5        )                                          γ          =                                    C              0                                      C              1                                      ,                                  ⁢                              …            ⁢                                                  ⁢            x                    =                                    C              x                                      C              0                                      ,                  y          =                                    C              y                                      C              0                                      ,                  c          =                                    C              c                                      C              0                                                          (        6        )                F(x,y,c): A normalization function representing a frequency deviation, when             1              2        ⁢                                  ⁢        γ              =    1    ,     that is, a capacitance swing=1.    Sx: A partially differentiated value of F(x,y,c) repersenting sensitivity of x (i.e., a frequency deviation per unit change).    γ: Capacitance ratio, x: Normalization variable capacitance 1,    y: Normalization variable capacitance 2, c: Oscillation circuit capacitance
FIG. 27 illustrates a simulation diagram of F(x, y, c) when c=25, y=20, 30, and 80, and x is variable. FIG. 28 is an enlarged diagram of the simulation diagram shown in FIG. 27. From the enlarged diagram, it is clear that sensitivity is high and a change in the normalization frequency deviation is large, in a region where x is small.
FIG. 29 illustrates the frequency deviation DL (x, y, c) and Sx when γ=200, that is, when the capacitance swing ½γ=2500 ppm. FIG. 30 illustrates a frequency deviation based on the DL when x=10. FIG. 31 illustrates a deviation from the DL curve when y=30 and x=10. In other words, FIG. 31 illustrates a difference of the DL relative to a value of x in a curve when y=30 and x=10, and a curve when y=80 and x=10, respectively. In other words, this diagram illustrates a deviation from a reference curve relative to the value of x, that is, interference. As is clear from the diagram, the deviation of each curve becomes larger when the value of x is smaller.
In FIG. 32, the ordinate represents a deviation from the reference curves shown in FIG. 31, and the abscissa represents a frequency deviation when y=30, based on X=10 shown in FIG. 30, that is, a value of the DL. Referring to FIG. 32, when the frequency decreases by 40 ppm in x (variable capacitance 1) and when the frequency increases by 30 ppm in y (variable capacitance 2), a distortion occurs by about 1 ppm. When the frequency decreases by 100 ppm, a distortion occurs by about 2.8 ppm.
The above distortions indicate that it is necessary to take into consideration the additional functions of the oscillator including two variable controllers, that is, addition of a temperature compensation function to an OCXO (high stability oscillator), addition of a temperature compensation function to a VCXO (voltage control oscillator), and addition of a voltage control function to a TCXO (temperature compensation oscillator).
The oscillator having two or more load capacitance variable functions to change the load capacitance of the external control function of frequency and the temperature compensation function as shown in FIG. 24 has a large problem in that mutual variable capacitance distort the variable quantity or the compensation quantity.
The oscillation frequency of the crystal oscillator can be changed according to three elements of load capacitance, environmental temperature, and oscillator current. Among these elements, the change of frequency according to the change in the load capacitance is used most. The high stability oscillator obtains higher stability by making constant the temperatures of the vibrator and the peripheral circuits. However, there is rare example of changing the frequency by using the vibrator current. Only a part of high stability oscillators is provided with a circuit that suppresses the vibrator current in order to improve aging.